Biological treatment is an important phase in the treatment of wastewaters and it is also the phase consuming most energy. In biological treatment carried out by means of aeration, it is necessary to continuously feed oxygen into the water in order to enable bacterial activity in the process. The oxygen concentration decreases also in natural waters during the long winter season, when the ice covering prevents a natural oxidation through the air. This phenomenon might lead to oxygen depletion, death of fish and a final pollution of the lake. Biological treatment is also used for e.g. removal of gases, such as radon, from the water by aeration.
Bottom aeration is the most common way for aeration of waters. In this method, air is pumped into the water through a membrane with holes on the bottom of the basin. Attempts have been made to start using surface aeration instead of bottom aeration, primarily of maintenance reasons. In these methods, air has been fed to rotating propellers on or near the surface of the water, which propellers have mixed air with streaming water. Also such devices are known, in which different flow pieces have been placed in the streams created by the propellers in order to improve the transfer of the air into the water and the mixing of the air with the water. The main drawback with these devices is their contamination and clogging in use. In addition, they have often required separate water mixers to keep the whole water content in the basin in a sufficient movement, whereby the waste water sludge can be prevented from falling down to the bottom of the basin.
Also such apparatuses have been used in aeration, wherein the water has been lead, by means of a pump, to a cylindrical space above the water. The cylindrical space has had a number of holes, through which the water has been ejected to the surface of the water.
In addition, ejectors have been used for aeration, in which a water jet has sucked air in itself and this jet has then been lead to the bottom of the basin, whereby circulation has been achieved in the basin at the same time. Characteristic for all surface aeration devices have been their efficiency, in other words, the amount of dissolved oxygen in each energy unit has been low.
The aeration causes ca 80% of the energy costs of a waste water treatment plant (i.e. an activated sludge plant, i.e. a plant using biological treatment) and this electricity consumption corresponds to ca 30% of the operational costs of the waste water treatment plant. The energy consumption is therefore of essential importance in aeration.
Typical oxidation effects in practice are, when it is question about a fine bubble aeration unit, which works as a bottom aerator: 1.7-3.0 kgO2/kWh, a coarse bubble aerator which works as a bottom aerator: 1.7-2.3 kgO2/kWh, a surface aerator: 1.3-2.2 kgO2/kWh or a combination aerator (OKI): 1.5-3.2 kgO2/kWh. The maintenance of a bottom aerator is difficult. Usually, the basin has to be emptied for the service (a part of them having separate bottom floats, which can be lifted one by one). In addition, the service is expensive and takes several weeks. For an example customer, the aeration system (including aeration plates, pipes, the installation, a compressor hall and the compressors) has cost 300.000 EUR. The service, which has to be performed with intervals of 4-6 years, costs 40-50.000 EUR.
In biological wastewater treatment there is often used both aerobic and anaerobic phases. The aerobic one requires oxidation and mixing, while the anaerobic one, in turn, requires an oxygen free environment and mixing. In the aerobic phase, biochemical load is typically removed from the wastewater and ammonium nitrogen is converted to nitrate by nitrification. In the anaerobic phase, the nitrogen in the wastewater is brought into a gaseous form and removes from the waste water. When the bottom aerators are stopped in the anaerobic phase, the water has to be mixed with separate mixers. This, in turn, increases the investments and number of objects to be maintained. In a part of the wastewater treatment plants, separate basins are used for the removal of nitrogen so that it would not be necessary to interrupt the bottom aeration. This, again, increases the investments. The bottom aerators gradually become clogged and their result worsens, especially if the aeration is often interrupted. By time, the effect of the bottom aerators lowers to 5-50% of the nominal effect.
Bottom aeration works best in basins 6-9 m deep, whereby the best efficiency is achieved. In low basins, the efficiency lowers fast, in very deep basins the compressed air has to be cooled to prevent the membranes of rubber/plastic from destruction. This increases both investments and operational costs.
The problem with surface aerators is the bad efficiency of the oxidation, the weak adjustability and the fact that the basin has to be rather low. A typical depth is 3.5 m, which makes the surface of the basins large and neither is their form optimal (expensive to make). A possibility to adjust would be important since the need of oxygen varies essentially e.g. when the load changes. Different surface aerators have been presented in e.g. the U.S. Pat. Nos. 5,021,154, 3,928,512 and 4,193,951.
The EP publication 465043 is also presented as prior art.